Automatic selection of receiving channels



Feb. 18, 1930. R. BowN AUTOMATIC SELECTION 0E RECEIVING CHANNELS FiledDeo. 11,* 192B lNvENToE Zown/ AT oRNEY g l E Vol-79295 Wazap 939.gif:

Patented Feb. 18, 1930 UNITED STATES PATENT ePrice RALPH BOIVN, 0FMAPIEWOOD, NEW JERSEY, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAIPHCOMPANY, A CORPORATION OF NEW YORK AUTOMATIC SELECTION 0F RECEIVINGCHANNELS Application filed December 11, 1928. Serial No. 325,150.

This invention relates to methods and means for limiting the extentwhich any received signal may vary in intensity.

@ne source of signal variation is that due to changes in the strength ofthe signal as received from the signal medium. In the case of radioreception such variations are generally known as fading. Fading isparticularly severe when receiving short wave lengths, and the methodgenerally employed for maintaining a more or less constant demodulatedsignal has been to provide an automatic gain control or regulator. Thismethod of regulation has certain limitations, however, and becomesinadequate, for example, when the signal drops below the limiting noiselevel. The present invention has as one of its obj ects the provision ofa system which will be free from these limitations and may be usedeither independently or supplementary to automatic gain regulation formaintaining a. constant output level at all times.

The invention is based on the fact that, if a number of receivingstations properly located and sufficiently far apart are used to pick upa transmitted signal, the signal does not fade similarly at each of thereceiving stations. Observations made on various wave lengths have shownthat the receiving antennae need only be separated from each other by adistance of the order of a wave length or so in order to get thiseffect. The possibility of the signal fading out equally at all of thereceiving stations at the same time is therefore reduced with increasein the number of receiving stations.

It has been heretofore proposed to use more than one receiver to pick upthe radio signal and to choose for use the receiver giving the bestresults. In the past this was accomplished by cutting over manually froma receiver giving a signal of low volume to another which at the momentwas giving a signal of larger volume, and again switching back to thefirst receiver or some other receiver as the signal again became low involume. Obviously, such a method of operation was limited to cases wherethe changes in the signal occurred slowly enough to permit the operatorto select the best receiver or receiver output.

VJ here the signal intensity changes quite rapidly, as is often the casewith short wave reception, a system capable at all times of rapidlycutting in the best of the available received signals is desirable. Itis therefore proposed to use a number of receiving stations, properlylocated, and to continuously and automatically select the receiver oroutput of the receiver giving a useful signal, minimizing in this waythe effect of rapid changes in the amplitude of the received current andreducing the possibility of the signal entirely fading out.

While it would be desirable to have the selecting apparatus so arrangedas to always select the receiver having the largest signal output,excellent reception will always be obtained if the selected receiver hasan output above a predetermined minimum. In accordance with the presentinvention, the selecting apparatus is arranged to select eX- clusivelythe first receiver whose output eX- ceeds the predetermined minimumvalue, thereby permitting of the use of a very simple form of selectingcircuit.

Where a particular receiver is so selected due to the fact that itsoutput as a whole is of sufficient volume to cause the operation ofl theselecting relay, it not infrequently happens that some parts of thevoice band from a particular receiver will be very low in volume, due toso-called selective fading. It is, therefore, desirable that for suchparts of the received signal reception take place from some otherreceiver. Accordingly, the voice band is divided up into severalsub-bands at each receiver by suitable filtering arrangements, and theselecting mechanism is so operated that reception for each sub-band isderived from a receiver whose output as to that subband eXc'eeds apredetermined minimum. In order to simplify the selecting operations,pilot frequencies corresponding to each subband are transmitted from thetransmitting station, and these pilot frequencies are used to operatethe selecting devices.

The invention may be more fully understood from the followingdescription when read in connection with the accompanying drawing,Figure l of which illustrates a circuit arrangement embodying theprinciples of the invention, and Fig. 2 of which is a curve illustratingthe operation of a selecting filter which may be employed in connectionwith the invention.

Referring to Fig. l of the drawing, 'Idesignates a radio transmitter ofsome known type capable of translating speech currents into radio wavesfor transmission through the ether. Preferably, transmitter T is of atype in which the so-called carrier component and one of the so-calledside band components are suppressed so that the entire energy capacityof the radio transmitter may be used for the transmission of signalingenergy in the form of the transmitted side band. In order to impress thespeech energy upon the radio transmitter, the speech circuit isconnected to the transmitter through a hybrid coil l0.

Inasmuch as the present invention involves theiseparation of the voiceband intoV a plurality of sub-bands at the receiver and the selection ofa receiver giving the signal of desired amplitude for each sub-band,provision is made at the transmitter to modulate Vthe carrier wave inaccordance with pilot frequencies, one corresponding to each sub-band.Accordingly, four pilot generators O1, O2, O3 and O.L are connected tothe midpoints of the hybrid coil 10 and the transmitter T is balanced bymeans of a network N so that while the pilot frequencies are impressedupon the transmitter T, they are prevented from reacting in the speechcircuit through the hybrid coil l0. Each pilot frequency should be solocated with respect to one of the sub-bands to which it corresponds asto be either at the I margin of the sub-band or within the subband sothat it will be subjected to substantially the same transmissionconditions as the sub-band to which it corresponds. Consequently, theside band transmitted through the air will include the four speechsub-bands together with radio frequencies corresponding to the severalpilot frequencies, and each of the pilot radio frequencies will -besubjected to substantially the saine transmission conditions as regardsfading, etc., as the side band frequencies corresponding to the speechsubbands.

At the receiving station a plurality of similar receivers Ra, Rb and RC1are provided. Each of these receivers is arranged to receive at the samewave length and each receiver may be of any type well known in the art,but where the carrier component is suppressed at the transmitter, thereceiver will be provided with local sources of energy to supply thecarrier wave which beats with the received side band to produce thedetected voice waves and impress them upon the output circuits La, Lband Le. As a result of the beating act-ion, audio frequency waves willalso be produced in the circuits La, Lb and Lc corresponding to each ofthe pilot frequencies generated at the transmitter. The amplitude ofthese pilot frequency waves will be determined by the amplitude of thelocally supplied carrier andthe amplitudes of those frequencies of thereceived side band which correspond to the original pilot frequencies.As the locally supplied carriers will have substantially the sameamplitude at each of the receivers, and as this amplitude will beentirely independent of 'fading or other transmission effects, the pilotfrequencies in the circuits La, Lb and IJc will have amplitudesdetermined by the fading effects of the corresponding radio frequenciesas received at the several receivers.

Now it is well known that where radio waves are subjected to fading, theeects are entirely different at different receivers in the same areaeven though the receivers are separated only by distances of the orderof a wave length or so. Also, a phenomenon known as selective fading hasbeen observed as a result of which certain parts of the detected voiceband will be subjected to fading effects, while other parts of the voiceband will either not be subjected to fading or will not fade to the sameextent. As regards the outputof a particular receiver, the detectedpilot frequencies will be subjected to the same fading as the voicesub-bands with* in which they lie. Consequently, any speech sub-bandwith its coresponding pilot frequency may be supplied to the outputcircuit of one receiver with one amplitude and to the outputs of otherreceivers with greater or lesser amplitude. The receiving circuit is,therefore, so organized that as respects each subband that sub-band willbe supplied to the final receiving circuit from a receiver in whoseoutput the sub-band appears with 'an amplitude above a predeterminedminimum. In order to control the selection between sub-bands fromdifferent receivers, the amplitudes of the pilot frequenciescorresponding to the different sub-bands determine the operation of theselecting devices, and as these pilot frequencies are receivedconstantly, the selection as respects each of the sub-bands takes placeindependently of whether or not voice signals are actually beingtransmitted and received. The selecting arrangement is of such characterthat in the final output of the receiving system the several sub-bandsmay not all be received `from the same receiver but `may be receivedfrom one or more receivers depending up-on the effects of selectivefading.

In order that the selecting operations may take place, the outputcircuit of each receiver, such as Ra, for example, is supplied to aplurality of individual circuits corresponding to each sub-band such asLM, Lga, L3a and L,... Band filters such as Fm, liga, F3, and F4a areincluded in the several branches to select in each branch differentsub-band of the detected voice band appearing in the output circuit ofthe receiver. These filters may be of any known character such, forexample, as filters of the well-known Campbell type. The circuits suchas Lm, L21, etc., may be connected to the common output circut L throughtransformers T13, Tm, T3a and TM.

In a similar manner, the output of receiver Rb is divided into fourbranches, L11), Lgb, etc., and these branches may be associated with thecommon output circuit L through transformersTlb, T213, TBb, etc. Each ofthese branches also includes sub-band selecting filters such as Fw,F21), etc.

So, also, the output of the receiver Re is divided into a plurality ofsub-band circuits Llc, Lge, etc., through sub-band filters Flc, F26,etc., and these sub-band circuits may be connectedv to the common outputcircuit L through transformers such as Tlc, TQQ, Tac, etc.

Vv here the pilot frequencies are located at the edge of thecorresponding sub-band the pilot frequencies may .be suppressed from thevarious sub-hand. circuits, such as Lm, L23, etc. by means of thesub-band filters such as F1a, F23, etc. If, however, the pilotfrequencies are not completely suppressed by this means, or if the pilotfrequences are so located as to fall well within the correspondingsub-band, they may be finally prevented from being transmitted to thehearer by including in the common circuit L a suppression filter ofknown type so arranged as to sharply suppress the unwanted pilotfrequencies while permitting the free transmission of the frequencieswithin the voice band other' than the pilot frequencies. Such a Altershould have a characteristic somewhat plitude to its associated voicesub-band, is

to control the selection of the sub-band from the output of a receiverin which the subband appears with an amplitude exceeding a predeterminedminimum limit. In order to accomplish this result, the pilot frequenciesare selected from the branch circuits, such as L12., Lga, etc., ahead ofthe band selecting filters F1a, Fm, etc., by means of sharply selectivecircuits such, for example, as tuned circuits, conventionally indicatedat Sla, Sm, S3., and Si., in the case of the receiver Ra. Similar pilotselecting circuits are provided for each of the other receivers. rlheselected pilot frequencies are impressed upon amplifier-detector unitssuch as Dm, EDM, etc., associated with each of the individual sub-bandcircuits. These amplifier-detector arrangements are indicatedonventionally and may be of any known type suoli, for example, as thoseused in connection with echo suppressors. The function of theamplifier-rectifiers is to amplify the selected pilot frequencies andproduce therefrom direct current components having amplitudes determinedby the amplitude of each pilot frequency appearing in the output of eachreceiver. The direct current components thus developed will correspondin amplitude to the amplitude of theV sub-hands appearing in the outputsof the several receivers. ln order to transmit over the various sub-bandcircuits, such as Lm, La, etc., only the direct current componentsresulting from the pilot frequencies, while suppressing the pilotfrequencies therefrom, selectingcircuits such as suppression filters Cm,Cgil, Ca, etc., areconnected upon the output sides of theamplifier-detectors. IThese selecting devices may be of any known typein the art and should be designed so as to permit the passage of directcurrents and perhaps currents of very low frequencies while suppressingthe higher frequency currents corresponding to the pilot frequencies.

rlhe direct currents thus generated and selected are transmitted overthe circuits such as Lla, Lm, Lw, etc., together with the selectedvoice, sub-bands. rfhe direct current components pass through thewindings of selecting relays Pla, Pga, etc., one corresponding to eachof the four sub-bands of the receiver Ba, and through the windings ofrelays Plb, P21), etc., corresponding to each of the four subbands inthe output of receiver Rb, and so on for other receivers, if desired. Inthe case illustrated, selecting relays are not provided for the severalsub-bands of the receiver Rc, which acts as a stand-by receiver andwhich is always connected to the common output when no other receiver isconnected thereto. rlhe direct currents generated in connection with thefour sub-band circuits of the receiver Re, therefore, in this instanceserve no useful purpose, and the apparatus for producing such directcurrents might he omi ted but is here illustrated in order to indicatehow the apparatus would be arranged if additional receivers were to beprovided beyond the receiver Re. The sub-band frequencies transmittedalong with the direct current components above referred to pass into thecom- 1 cy f1. The arrangement is such that the first operated relay fromthe left in each group determines the receiver from Which the subbandcorresponding to the pilot frequency f1, for example, is to be derived.In order to accomplish this result, the armature of each relay has itscontacts so arranged that When the relay is non-energized thetransformers,

such as Tm, T11 etc., are short-circuited by the back contacts of theselecting relays. Therefore, if all of the relays are released, thesub-band corresponding to the pilot frequency f1 Will be derived fromthe receiver Re through the transformer Tlc. If, however, one of therelays as, for example, relay P11, is operated, the short-circuit isremoved from the secondary of its transformer T11, and all transformersto the right are short circuited over the front Contact of said relayand all transformers to the left are short-circuited over the backcontacts of the non-operated relays. The result is that as respects thesub-band corresponding to pilot frequency f1, the output of the receiverR1, is exclusively connected to the circuit L.

In order to more fully understand the selecting operation, let us assumea concrete case in Which the sub-band corresponding to pilot frequencyf1 appears with an amplitude above the predetermined minimum in theoutputs of all three receivers, the sub-band corsponding to pilotfrequency f2 appears With an amplitude above the minimum in the outputsof the receivers R1, and Re, the sub-band corresponding to the pilotfrequency f3 appears With an amplitude above the minimum in the outputsof receivers R1, and Rc, While the sub-band corresponding to the pilotfrequency f1 appears with an amplitude above the minimum in the outputof receiver R1,

f only. In each of these cases the pilot frequencies corresponding tothe several sub-bands Will appear in the outputs of the severalreceivers With amplitudes proportional to those of the sub-bands towhich they correspond. Therefore, in the case of the first sub-bandhaving the pilot frequency f1, this pilot frequency Will appear in theoutputs of all three receivers With an amplitude above a predeterminedminimum Which is necessary to provide the direct current component foroperating the selecting relays. (It Will be understood, of course, thatthe relays Will be so designed as to have a margin of operationdetermined by the minimum signal component which it is desired toreceive.) Under the conditions just described, the direct currents willbe produced from the pilot frequency f1 in the outputs of each of thethree receivers which are of suficient amplitude to operate all of theselecting relays of the f1 group` Consequently, both relays P11 and P11,Will be operated, with the result that the second-- ary of thetransformer Tla Will have its shortcircuit opened, While thetransformers T11, and T1c will be short-circuited over the front contactof relay P111, even though the shortcircuit normally existing about theWinding of transformer T11, is removed. Consequently, as regards thesub-band corresponding to the pilot frequency f1, this sub-band isderived exclusively from the output of the receiver P., over the circuitL1a.

In the assumed case, the pilot frequency f2 will appear in the outputsof receivers R1, and RC With suflicient amplitude so that thecorresponding direct current components Will operate relay P11, over thebranch L21, Without causing the operation of relay P21. The secondary oftransformer T21, therefore, remains short-circuited and the transformerT2c is short-circuited over the front cont-act of relay P21, which, byremoving the shortcircuit normally existing across the secondary of thetransformer T21, causes the subhand corresponding to pilot frequency f2to be derived over the circuit L21, from the receiver R1, exclusively.

Similarly, in the case of the sub-band corresponding to frequency f3,the pilot frequency f3 will appear in the output circuits of thereceivers R, and R-c With amplitudes above the minimum necessary toproduce direct currents capable of operating the selecting relays P3,1(there being no relay corresponding to the receiver R1, for thissub-band) The relay P311 upon being operated, removes the short-circuitfrom the secondary of transformer T3a and at its front contactshortcircuits transformers T11, and T3c so that the third side band issupplied to the common receiving circuit L over the branch circuit L3,from receiver R1 exclusively.

Finally, as respects the fourth sub-band which is received With anamplitude above the predetermined minimum in the output of the receiverRe, only the pilot frequency f1 will appear in the output of thereceiver Rc With sulicient amplitude to operate a selecting relay of thef4 group, if such relay is provided. In the case illustrated, no suchrelay is provided, and as the frequency f1 appears in the outputcircuits of receivers R11 and R1, with insuflicient amplitude to causethe operation of selecting relays P41 and P111, the fourth sub-band willbe supplied to the common circuit L from the output of receiver Re overthe branch L4, and through the transformer Tic. Incidentally, it mightbe noted at this point that the fourth sub-band would be suppliedthrough the transformer Tic, even if no one of these receivers suppliedthis subband ivi h an amplitude above the required minimum. The receiverRe functions, as regards this sub-band in the instance assumed, as astand-by receiver.

To summarize for the assumed case, the first and third sub-bands arederived exclusively from the receiver Ba, the second vsubband from thereceiver Rh and the fourth sub-band exclusively from the receiver Re.Obviously, various other combinations Would arise depending upon theselective fading conditions affecting the several sub-bandslt will beobvious that the general princiv, ples herein disclosed may be embodiedin many other organizations Widely dierent from those illustratedwithout departing from the spirit of the invention as dened in thefollowing claims.

What is claimed is:

l. In a signaling system subject to variable transmission,conditions, aplurality of receivers each receiving the same signal band and sorelated to each other that the received signal bands and component partsthereof vary differently at the several receivers With variations intransmission conditions, means to separate the received bands at eachreceiver into sub-bands, a receiving circuit, and means to combine insaid receiving circuit sub-bands each selected from a receiver Whoseoutput of that sub-band exceeds a predetermined minimum.

2. In a signaling system subject to variable transmission conditions, aplurality of receivers each receiving the same signal band and sorelated to each other that the received signal bands and component partsthereof vary dierently at the several receivers with variations intransmission conditions, means to separate the received bands at eachreceiver into sub-bands, a receivinO circuit, means to select as to eachsub-band the receiver Whose output of that sub-band exceeds apredetermined minimum, and means to combine in said receiving circuitthe selected sub-bands from the selected receivers.

3. n a signaling system subject to variable transmission conditions, a.plurality of receivers each receiving the same signal band and sorelated to each other that the received signal bands and component partsthereof vary difi ferently at the several receivers With variations intransmission conditions, means to separate the received bands at eachreceiver into sub-bands, a receiving circuit, separate connections tosaid receiving circuit adapted to be establshed for the transmission ofeach means to establish an exclusive connection for each sub-band to areceiver Whose output of that sub-band exceeds a predetermined minimum.Y

4. In a signaling system subject to variable transmission conditions, aplurality of receivers each receiving the same signal band and sorelated to each other that the received signal bands and component partsthereof vary differently at the several receivers with variations intransmission conditions, means to separate the received bands at eachreceiver into subbands, a receiving circuit, separate connections tosaid receiving circuit adapted to be established for the transmission ofeach separate sub-band from each receiver, and means to complete one ofsaid connections to a receiver for each sub-band to the exclusion ofother connections for the same sub-hand.

5. In a signaling system subject to variable transmission conditions, aplurality of receivers each receiving the same signal band and sorelated to each other that the received signal bands and component partsthereof vary differently at the several receivers With variations intransmission conditions, means to separate the received bands at eachreceiver into sub-bands, a receiving circuit, separate connections tosaid receiving circuit adapted to be established for the transmission ofeach separate sub-band from each receiver, and means to complete foreach sub-band a connection to a receiver Whose output of that subbandexceeds a predetermined minimum, said completed connection for eachsub-hand being exclusive of connections to other receivers for the samesub-band.

6. In a signaling system subject to variable conditions, a plurality ofreceivers each receiving the same signal band and so related to eachother that the received signal bands and component parts thereof varydifferently at the several receivers With variations in transmissionconditions, means to separate the received bands at each receiver intosubbands, a receiving circuit, means to transmit a pilot frequencycorresponding to each subband from a transmitting station, separateconnections to said receiving circuit adapted to be established for thetransmission of each separate subeband for each receiver, and meanscontrolled by the pilot frequency for each sub-band to complete one ofsaid con nections' to a receiver for each sub-band.

7. In a signaling system subject to variable transmission conditions, aplurality of receivers each receiving the same signal band and sorelated to each other that the received signal bands and component partsthereof vary differently at the several receivers With variations intransmission conditions, means to separate the received bands at eachreceiver into sub-bands, a receiving circuit, means to transmit a pilotfrequency corresponding to each sub-band from a transmitting station,separate connections to said receiving circuit adapted to be establishedfor the transmission of each separate sub-band for each receiver, andmeans controlled by the pilot frequency for each subdoand to complete aconnection for each sub-band to a receiver Whose output of such sub-bandexceeds a predetermined minimum.

In testimony whereof, I have signeds my name t0 this specification this7th day of December, 1928.

RALPH BOWN.

